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doc/sql.extensions/README.aggregate_tracking

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+SQL Language Extension: Improved aggregate handling
+
+Syntax
+========
+
+
+GROUP BY
+--------
+SELECT ... FROM .... [GROUP BY group_by_list]
+
+group_by_list : group_by_item [, group_by_list];
+
+group_by_item : column_name
+		| ordinal
+		| udf
+		| group_by_function;
+
+group_by_function	: numeric_value_function
+		| string_value_function
+		| case_expression
+		;
+
+numeric_value_function	: EXTRACT '(' timestamp_part FROM value ')';
+
+string_value_function	:  SUBSTRING '(' value FROM pos_short_integer ')'
+			| SUBSTRING '(' value FROM pos_short_integer FOR nonneg_short_integer ')'
+			| KW_UPPER '(' value ')'
+			;
+
+(FB1.0 only allowed GROUP BY column_name and udf)
+Ordinal references to a n-th select-item from the select-list (same as ORDER BY).
+The group_by_item cannot reference to any aggregate-function (also not burried 
+inside a expression) from the same context.
+
+
+HAVING
+--------
+The having clause only allows aggregate functions or expressions 
+that are part of the GROUP BY clause. Previously it was allowed
+to use columns that were not part of the GROUP BY clause.
+
+
+ORDER BY
+--------
+When the context is an aggregate statement then the ORDER BY 
+clause only allows valid expressions. That are aggregate 
+functions or expression part of the GROUP BY clause.
+Previously it was allowed to use non-valid expressions.
+
+
+Aggregate functions inside sub-selects
+--------
+It is now possible to use a aggregate function or expression
+contained in the GROUP BY clause inside a sub-select.
+(See Examples A)
+
+
+Mixing aggregate functions from different contexts
+--------
+You can use aggregate functions from different contexts inside 
+a expression.
+(See Example B)
+
+
+Sub-selects are supported inside a aggregate function
+--------
+Using a singleton select expression inside a aggregate function is
+supported.
+(See Example C)
+
+
+Nested aggregate functions
+--------
+Using a aggregate function inside a other aggregate function is
+possible if the aggregate function inside is from a lower context.
+(See Example C)
+
+
+Author:
+    Arno Brinkman <firebird@abvisie.nl>
+
+
+N O T E S
+=========
+
+ - Not all expressions are currently alowed inside the GROUP BY list.
+   (concatenation for example) 
+ - ORDER BY clause only accepts valid expressions (this was not for FB1.0)
+ - HAVING clause only accepts valid expressions (this was not for FB1.0)
+ - Using ordinal 'copies' the expression from the select list as does the
+   order by clause. This means when a ordinal references to a sub-select
+   the sub-select is at least executed twice.
+
+
+Examples
+========
+
+
+A)
+  SELECT
+    r.RDB$RELATION_NAME,
+    MAX(r.RDB$FIELD_POSITION),
+    (SELECT 
+       r2.RDB$FIELD_NAME 
+     FROM 
+       RDB$RELATION_FIELDS r2
+     WHERE 
+       r2.RDB$RELATION_NAME = r.RDB$RELATION_NAME and
+       r2.RDB$FIELD_POSITION = MAX(r.RDB$FIELD_POSITION))
+  FROM
+    RDB$RELATION_FIELDS r
+  GROUP BY
+    1
+
+  SELECT
+    rf.RDB$RELATION_NAME AS "Relationname",
+    (SELECT
+       r.RDB$RELATION_ID
+     FROM
+       RDB$RELATIONS r
+     WHERE
+       r.RDB$RELATION_NAME = rf.RDB$RELATION_NAME) AS "ID",
+    COUNT(*) AS "Fields"
+  FROM
+    RDB$RELATION_FIELDS rf
+  GROUP BY
+    rf.RDB$RELATION_NAME
+
+
+B)
+  SELECT
+    r.RDB$RELATION_NAME,
+    MAX(i.RDB$STATISTICS) AS "Max1",
+    (SELECT
+       COUNT(*) || ' - ' || MAX(i.RDB$STATISTICS)
+     FROM
+       RDB$RELATION_FIELDS rf
+     WHERE
+       rf.RDB$RELATION_NAME = r.RDB$RELATION_NAME) AS "Max2"
+  FROM
+    RDB$RELATIONS r
+    JOIN RDB$INDICES i on (i.RDB$RELATION_NAME = r.RDB$RELATION_NAME)
+  GROUP BY
+    r.RDB$RELATION_NAME
+  HAVING
+    MIN(i.RDB$STATISTICS) <> MAX(i.RDB$STATISTICS)
+
+Note! This query gives results in FB1.0, but they are WRONG!
+
+
+C)
+ SELECT
+    r.RDB$RELATION_NAME,
+    SUM((SELECT
+           COUNT(*)
+         FROM
+           RDB$RELATION_FIELDS rf
+         WHERE
+           rf.RDB$RELATION_NAME = r.RDB$RELATION_NAME))
+  FROM
+    RDB$RELATIONS r
+    JOIN RDB$INDICES i on (i.RDB$RELATION_NAME = r.RDB$RELATION_NAME)
+  GROUP BY
+    r.RDB$RELATION_NAME
+
+